Resolve "Condev topic renamed without redirect"

What does this MR do?

Adds a convdev.md file to the doc/user/instance_statistics directory. As noted in #197873 (closed), the file was there for 12.5. It was renamed for 12.6, but without following the Changing Document Location guidance for such situations.

This MR addresses that error.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #197873 (closed)